The advantages of using a combination anchor line consisting of a lower length of chain cable serially connected to an upper length of wire rope in the mooring of offshore drill platforms and the like are well known. In particular, better anchoring characteristics at certain water depths can be achieved than otherwise possible through use of wire rope or chain alone, and an overall capability of mooring in deeper waters is obtained. Use of combination anchor lines, however, introduced several problems not encountered in prior systems.
In particular, it is clearly desirable to haul in and pay out the anchor line to the extent possible from the deck of the anchored structure, using winches and windlasses appropriately mounted on the structure. Such operation requires, however, the conveying of a wire rope-chain cable connector over a fairlead sheave attached to the structure without unduly bending the wire rope. In original combination anchoring systems the problem of conveying the junction of the wire rope and chain over a fairlead sheave was avoided by breaking the wire rope-chain cable connection on the deck of a workboat, accompanying the floating structure, and either storing the chain cable in the workboat or conveying the chain cable for storage to the deck of the floating structure without engaging the fairlead sheave. Such practice was inconvenient and hazardous in rough waters.
In U.S. Pat. No. 3,842,776 which issued to Wudtke on Oct. 22, 1974, an anchoring system is described in which a special wire rope-chain cable connector serves to convey the junction between wire rope and chain cable over a fairlead sheave. The connector is formed with an enlarged part-spherical end which is conveyed in a mating outermost groove of a special fairlead sheave during transition from wire rope to chain cable. The point of connection of the chain end link with the connector is sufficiently close to the rotational centre of the connector's part-spherical surface and the connector is sufficiently distant from the hub of the sheave that the rope end of the connector pivots freely in response to rope movement, thereby reducing rope bending. Although successfully implemented, such a system has the notable disadvantage that the special fairlead sheave must be considerably larger than a convention fairlead sheave, and consequently very cumbersome and heavy.
It is an object of the present invention to provide an anchoring system which conveys wire rope and chain cable over a fairlead sheave without undue bending of the wire rope and which does not require an oversized fairlead sheave.